Operation Sea Lion could not end with troops reaching a beach; Germany first had to defeat the RAF and then keep control of the Channel long enough to reinforce and supply them. Royal Navy losses accepted during the evacuations from Dunkirk, Norway, Greece, and Crete indicate what the fleet might risk to stop an invasion of Britain itself. Coastal guns, Home Guard positions, regular troops, and successive stop lines were meant to slow the landing until mobile formations could concentrate. The decisive defensive idea was to sever the beachhead from the sea and destroy it before German supply could become secure.
Notable Point · 57:07-1:00:36
Sea Lion required air superiority and lasting control of the Channel
Germany needed air and sea control long enough to sustain a landing, while Britain's layered defenses bought time for mobile forces and the fleet to cut the invasion off.